The present invention generally relates to copper-filled vias and lines in ceramic substrates and, more particularly, to a copper-based paste containing refractory metal additions for densification control of the via and thick film lines produced by sintering.
The use of copper-filled vias in ceramic substrates, particularly multilayer ceramic substrates, and sintering processes for producing them are well known in the semiconductor packaging art as taught, for example, in Herron et al. U.S. Pat. No. 4,234,367, the disclosure of which is incorporated by reference herein. Recently, more interest has been focused on the associated problems of the disparity in shrinkage rates between copper and ceramic as well as the onset of via "opens", particularly as via diameters are reduced below 100 microns in high circuit density applications. A discussion of such problems is given in Herron et al., U.S. Pat. No. 4,776,978, the disclosure of which is incorporated by reference herein.
As set forth in the cited No. 4,776,978 patent, metal particles, such as copper, in the via and line paste undergo sintering while also experiencing shrinkage during the initial phase of the sintering cycle whereas the ceramic and/or glass particles (of the bulk ceramic substrate containing the vias and lines) undergo sintering during the intermediate and final phases of the sintering cycle along with their characteristic shrinkage. One method of delaying the onset of sintering of the metal particles until at least the intermediate phase of the sintering cycle is to intersperse the metal particles in the vias and lines with a high melting point material such as aluminum oxide.
Although the foregoing generalized considerations have been known in the art for some time and have provided the basis for techniques for overcoming previous shrinkage and related problems, more refined and detailed approaches are required to meet the needs of copper-filled vias and lines in ceramic substrates with increasing circuit densities and the concomitant via diameters in the range of about 85 to 100 microns. It is also desirable to provide a copper paste mixture which can be adapted for use with the next generation of ceramic packages which exhibit reduced shrinkage from sintering.
The following references illustrate previous techniques attempting to overcome shrinkage and other problems.
Brownlow et al., U.S. Pat. No. 4,599,277 discloses the addition of an organometallic compound to a metal member such as copper paste which compound undergoes decomposition during sintering to provide a coating such as aluminum oxide on the copper particles towards obtaining better shrinkage match between copper and the bulk ceramic substrate during sintering.
Beil, U.S. Pat. No. 4,020,206 discloses a low shrinkage via paste made from gold particles and refractory particles such as carbides, oxides, and quartz.
Herron et al., U.S. Pat. No. 4,671,928 discloses that molybdenum or aluminum oxide may be added to the copper paste to retard sintering and reduce densification
Other references generally illustrate the formation of carbonyl coatings on metal particles. Thus, Goodrich et al., U.S. Pat. No. 3,342,587 disclose the deposition of nickel and iron carbonyls on copper particles. Frey et al., U.S. Pat. No. 3,549,412 and Jenkin, U.S. Pat. No. 4,606,941 disclose the formation of chromium, molybdenum, tantalum, and tungsten carbonyls on metals in general.
One purpose of the present invention is to provide a copper paste with a refractory metal addition to substantially match the shrinkage of the resulting copper material to the shrinkage, if any, during sintering of the bulk ceramic substrate.
Another purpose of the present invention is to provide a copper paste with a refractory metal addition to provide substantially reduced shrinkage after sintering of the copper material in low shrinkage porous ceramic substrates.
These and other purposes of the present invention will become more apparent after referring to the following specification.